The invention relates to a charging circuit for charging, from a voltage source, a pulse-forming network to a predetermined voltage.
Such a circuit can be applied in a radar transmitter for the generation of radar transmit pulses. The radar transmitter will in that case be provided with a high-power radar transmitter tube, such as a crossfield amplifier, a travelling wave tube or a klystron, whose cathode must be brought to a pulsed high voltage. The high-voltage pulses are generated by a high-voltage transformer, the primary winding of which is connected to a low-voltage pulse generator, consisting of at least a pulse-forming network and a switching element.
As regards applications where a radar system's only task is to detect moving objects, heavy demands are made on the reproducibility of the high-voltage pulses, applied to the radar transmitter tube. As a consequence of minor variations in the high-voltage pulses, stationary objects also seem to move, which may easily cause an overload on the equipment, processing the target echoes, received by the radar system.
The reproducibility of the high-voltage pulses is to a considerable extent determined by the reproducibility of the charging voltage of the pulse-forming network. The charge is usually provided by a buffer capacitor, which is powered from the mains via a rectifier circuit.
The charging voltage of the buffer capacitor is not constant, it varies with the mains voltage and is often afflicted with a ripple voltage, originating from the rectifier circuit.
A method well-known in the art for charging a pulse-forming network is the resonant charging through an inductor. If the pulse-forming network has reached the required voltage, the charging current is interrupted and the residual energy stored in the inductor is fed back via a circuit which is connected to the inductor by way of secondary winding. At that moment the inductor is in fact a transformer.
The drawback of this method is that the residual energy in one winding of a transformer can never be completely drained via the other winding of that transformer, because of the leakage self-inductance of the transformer. As a consequence, the pulse-forming network will receive some additional charge from the primary winding of the transformer after the interruption of the actual charging current. Moreover, the amount of additional charge is a function of the instantaneous charging voltage of the buffer capacitor, which means that the reproducibility of the charging voltage of the pulse-forming network is adversely affected.
A conventional circuit of the resonant type is schematically represented in FIG. 1. A buffer capacitor 1 is charged from the mains voltage. Under the control of a control unit 2, switching element 3 closes, as a result of which current can flow via an inductor 4 to a pulse-forming network 5, in this case represented by a capacitor. In reality, pulse-forming network 5 is also provided with inductors, but these hardly play any role during charging. The voltage on the pulse-forming network 5 is applied to control unit 2 and when this voltage has reached a predetermined value V, switching element 3 is opened under the control of control unit 2. At this moment there will still be a current I flowing through inductor 4. Via a diode 6 this current can continue to flow after the opening of switching element 3. Eventually the current will cease to flow, but the voltage on the pulse-forming network has meanwhile increased according to the formula: EQU 1/2C(V').sup.2 =1/2CV.sup.2 +1/2LI.sup.2,
where C is the capacitance of the pulse-forming network 5, L is the self-inductance of inductor 4, V is the voltage on the pulse-forming network during the opening of switching element 3, I is the current through the inductor 4 during the opening of switching element 3 and V' is the ultimate voltage on the pulse-forming network; this on the assumption that the capacitance of buffer capacitor 1 is very much larger than the capacitance of pulse-forming network 5. The ultimate voltage V' depends on I and consequently on the instantaneous voltage on buffer capacitor 1. This voltage in particular strongly fluctuates, as a consequence of mains fluctuations and a continuously present ripple voltage, caused by the rectification of the mains voltage.
A conventional charging circuit of the resonant type is represented in FIG. 2. Here again, switching element 3 closes under the control of the control unit 2, as a result of which a current can flow to pulse-forming network 5 via a primary winding 7A of a transformer 7. A switching element 8 is in open condition, so that no current can flow in a secondary winding 7B of transformer 7. In this condition the primary winding of transformer 7 actually forms an inductor, so that the behaviour of the circuit is identical to that of the circuit which is schematically represented in FIG. 1. When the voltage on pulse-forming network 5 has reached a predetermined value V, witching element 3 is opened under the control of control unit 2. At the same time, however, switching element 8 is closed, as a result of which energy, stored in the magnetic field of transformer 7, is fed back to buffer capacitor 1 via a current through the secondary winding, a diode 9 and switching element 8. If the magnetic coupling between the primary winding 7A and the secondary winding 7B of transformer 7 is perfect, the condition can be attained that no more energy is supplied to pulse-forming network 5 after the opening of switching element 3. In the event of a less perfect coupling the following applies again: EQU 1/2C(V').sup.2 =1/2CV.sup.2 +1/2LI.sup.2,
L now representing however the leakage self-inductance of transformer 7. As compared with the simple charging circuit, as schematically represented in FIG. 1, there is an improvement of orders of magnitude in the reproducibility of the charging voltage of the pulse-forming network.